The ability to detect when a called-party answers the telephone is a requirement for many applications and services that are now available throughout telephone networks, such as automatic voice information, voice messaging, telemarketing, etc . . . If the beginning and the end of called-party's introductory message after hang-up are accurately detected, efficiency in the delivered message will be optimized. Such a detection however is technically difficult to achieve, and it is currently estimated that inaccurate answer detection still costs telephone carriers and users a huge amount of money every year.
U.S. Pat. No. 4,672,669 filed on May 31, 1984, describes a Voice Activity Detector featuring a determination algorithm comprising two steps: checking the received signal energy versus an adaptive threshold, and testing the rate of change of spectral parameters (autocorrelation coefficients) for a number of consecutive 20ms sequences of the received signal versus a second fixed threshold.
The Detector was designed to take advantage of the half duplex effect of telephone conversation, and interpolate additional talkers up to twice the overall channel capacity (multiple telephone channels application). It was then able to detect even low-level "speech" segments in high-level uncorrelated or correlated background noise, but not in the presence of tones, including call progress tones (Detector designed for already established communications).
To detect voice on a communication line, even in the case of tones presence, it can still be considered to use spectral parameters thru a number of consecutive 20ms sequences, but it might happen that variations of these autocorrelation coefficients for some tone signals (including ring-back, SIT tones, . . .), due to the difference between the tone period and the 20ms computation window length, are such that the second threshold is difficult to establish, if not impossible. Moreover, in case of voiced speech (speech `sounding` like a tone) the method implemented in the Voice Activity Detector appears to not be sensitive enough.